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I'm a Realtor, but in MA. My understanding is that only the people listed on the lease may be living "full time" in the apartment. What constitutes full time might be spelled out in the lease, but you might find the answer to that in google. Search for NJ rental laws. The laws differ from state to state.

Only the people listed on the lease are to live in the apartment. Anything else is a violation o the lease.
Most municipalities have regulations on the number of people allowed to reside in a dwelling, by size, number of bedrooms and by the age of the occupants.
You should be able to call 211 to get that information for your area.
Let's say at present you and a friend are sharing the apartment. Your friend wants her boyfriend to move in. In order to legally do so he has to be put on the lease, not necessarily as a lessee but as an authorized occupant. It would be the same if she had a baby. There is a limit to how many are allowed to live in the space as well.